It is well known that silver halide emulsion preparation includes the stages of nucleation and growth of the silver halide grains within a continuous phase. In the nucleation stage, new grains of very small size are created. The growth stage involves addition of new material to the existing small grains to make them larger.
Both the nucleation and growth stages of silver halide emulsion preparation require the use of a peptizer to avoid the coalescence or flocculation of the silver halide grains and to control grain growth and dispersity. The coated emulsion contains a vehicle that typically includes a colloidal binder (or mixtures of binders) and the peptizer. Characteristics of grain growth and dispersity have a critical effect on sensitometric and photographic properties of the resulting emulsions.
While a variety of hydrophilic colloids are known to be useful as peptizers, the most commonly employed peptizers are gelatins, such as alkali-treated gelatin (bovine bone or hide gelatin) or acid-treated gelatin (pigskin or bovine bone gelatin), and gelatin derivatives, such as acetylated and phthalated gelatins. Such peptizers are collectively known as "gelatino-peptizers".
These same materials can also be used as the binders in the photographic emulsions. However, there are many similar materials that are useful as binders but which are ineffective as peptizers.
Gelatins generally contain various impurities and comprise a wide variety of different types of protein and polypeptide molecules in various configurations and sizes. The resulting lack of uniformity from batch to batch has been a recognized problem for a long time, and various researchers have valiantly attempted to modify or replace gelatins to provide more uniform peptizers or binders. Removal of impurities by various means is a common practice in order to produce photographically acceptable gelatins.
Synthetic copolymers containing sulfide groups have been prepared to replace gelatin as a peptizer (see U.S. Pat. No. 3,615,624 of Smith et al). Synthetic gelatin replacements have also been prepared as polyimide condensation products of methionine and an .alpha.-amino carboxylic acid (see U.S. Pat. No. 4,315,072 of Fox et al).
Naturally occurring gelatins have been treated in a variety of ways to modify its peptizing and sensitometric properties. For example, U.S. Pat. No. 4,713,320 (Maskasky) describes lowering the content of methionine in the gelatin by oxidation in order to make thin tabular silver halide grains having certain characteristics. Alkylation of methionine groups in gelatin is described in U.S. Pat. No. 4,942,120 (King et al) for similar purposes. Use of gelatin with low cysteine content is described in U.S. Pat. No. 4,990,440 (Moll et al).
More recently, polypeptides made by recombinant DNA techniques are described as coating vehicle replacements for type I collagen of conventional gelatin for use in holography (see FR-A-2,685,347, Obrecht et al). The described polypeptides were expressed in E. coli and recovered using conventional techniques and nickel-NTA-agarose resins. The noted polypeptides have specific amino acid characteristics. The specific peptides are rich in Gly-Pro-Ala and Gly-Glu-Arg triplets, and also contain a triplet of histidines to provide affinity for the capture resins. A methionine is included between the histidine and non-histidine triplets to enable chemical degradation with cyanogen bromide and release of the desired polypeptide. A cysteine is included for binding to chromatographic resins or proteins, and a leucine is placed critically between the methionine and histidine because of its restriction site. Thus, the polypeptides have a complicated sequence of amino acids, particularly on one end, for capture and recovery of the desired material. The actual usefulness of the described materials is not demonstrated in the noted publication.
Such materials are believed to have a low expression yield. That is, often the level of expression is so low as to be detectable only using radioactive labeling of the cells, or antibodies specific to a particular tag or peptide sequence. The preparation of these materials is lengthy and tedious, requiring a costly purification procedure from cell paste lysates. Because of the low expression level, the production yield is also low. Because of the particular described capture mode, the removal of epitopic affinity tags by chemical or enzymatic means is tedious and costly with no certainty of complete removal. Thus, some of the molecules have extraneous amino acids that are not part of the desired collagen-like sequence.
It would be useful to have a way to prepare photographic emulsions using highly uniform and easily prepared collagen-type peptizers. It would be particularly useful to be able to control grain morphology with particular peptizers so that emulsions could be prepared having specific types of silver halide grains.